Our invention relates to ceramic materials in general and, in particular, to semiconductive ceramic materials composed principally of strontium calcium titanate, Sr.sub.(1-x) Ca.sub.x TiO.sub.3, and to a process for the fabrication of coherent bodies of such ceramic materials. Having a voltage-dependent nonlinear resistance, the ceramic materials according to our invention find a typical application in varistors, known also as voltage-dependent resistors, enabling the same to function both as such and as capacitors.
Parallel connections of varistors and capacitors have been used extensively for the absorption or suppression of abnormal voltages in electronic circuits. It is obvious, then, that devices possessing both varistor and capacitor functions can attain the same purpose with simpler circuit configurations. Varistors capable of such a dual function have been suggested by European Patent Publication No. 44,981 filed by Taiyo Yuden Co., Ltd., and dated Feb. 3, 1982.
The published European patent application proposes semiconductive ceramic materials in a perovskite structure composed primarily of strontium titanate, SrTiO.sub.3. The nonlinear volt-ampere characteristic of the materials makes them well suited for use in the dual-purpose varistors which are capable of single-handedly suppressing abnormal voltages. Although satisfactory for most practical purposes, the varistors in accordance with the prior art ceramic compositions have proved to have some properties in need of improvement. One of these is the temperature dependency of the varistor voltage (i.e., in the volt-ampere curve of the varistor, the voltage at which the current starts rising in magnitude). Another is the capability of withstanding voltage and current surges in use.